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A semiochemical (from Greek semeon meaning "signal") is a generic term used for a chemical substance or mixture that carries a message for purpose of communication. Semiochemical communication can be divided into two broad classes: communication between individuals of the same species (intraspecific) or communication between different species (interspecific).
Many insects, including parasitic insects, use semiochemicals, which are natural chemicals released by an organism that affect the behaviors of other individuals. Pheromones are intraspecific signals that aid in finding mates, food and habitat resources, warning of enemies, and avoiding competition. Interspecific signals known as allomones and kairomones have similar functions.
A pheromone (from Greek phero "to bear" + hormone from Greek - "impetus") is a secreted or excreted chemical factor that triggers a social response in members of the same species. Pheromones are chemicals capable of acting outside the body of the secreting individual to impact the behavior of the receiving individual. There are alarm pheromones, food trail pheromones, sex pheromones, and many others that affect behavior or physiology. Their use among insects has been particularly well documented. In addition, some vertebrates and plants communicate by using pheromones. A notable example of pheromone usage to indicate sexual receptivity in insects can be seen in the female Dawson's Burrowing bee, which uses a particular mixture of cuticular hydrocarbons to signal sexual receptivity to mating, and then another mixture to indicate sexual disinterest. These hydrocarbons, in association with other chemical signals produced in the Dufour's gland, have been implicated in male repulsion signaling as well. 
The term "pheromone" was introduced by Peter Karlson and Martin Lüscher in 1959, based on the Greek word pherein (to transport) and hormone (to stimulate). They are also sometimes classified as ecto-hormones. German Biochemist Adolf Butenandt characterized the first such chemical, Bombykol (a chemically well-characterized pheromone released by the female silkworm to attract mates).
An allomone is any chemical substance produced and released by an individual of one species that affects the behaviour of a member of another species to the benefit of the originator but not the receiver. Production of allomones is a common form of defence, such as by plant species against insect herbivores or prey species against predators. Sometimes species produce the sex pheromones of the organisms they exploit as prey or pollinators (such as bolas spiders and some orchids). "Allomone" was proposed by Brown, Eisner, and Whittaker to denote those substances which convey an advantage upon the emitter.
A kairomone is a semiochemical, emitted by an organism, which mediates interspecific interactions in a way that benefits an individual of another species which receives it, without benefitting the emitter. Two main ecological cues are provided by kairomones; they generally either indicate a food source for the receiver, or give warning of the presence of a predator. Often a pheromone may be utilized as a kairomone by a predator or parasitoid to locate the emitting organism
The goals of using semiochemicals in pest control are
- to monitor pest populations to determine if control is warranted and
- to alter the behavior of the pest or its enemies to the detriment of the pest. In general, the advantages of using semiochemicals are
- they have adverse effects only on target pests,
- they are relatively nontoxic and required in low amounts,
- they are nonpersistent and environmentally safe
- they appear difficult for insects to develop resistance against. Monitoring of pest populations with pheromones is often integrated in management programs.
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